Influences of the number and length of longitudinal fins on the single and cyclic charging and discharging performance of vertical double-tube latent heat storage systems

Using longitudinal fins is one of the efficient ways to enhance the heat transfer performance in phase change materials (PCM) based latent heat thermal energy storage systems. However, the previous studies do not consider the influence of the fins' geometric parameters on the single and cyclic charging and discharging performance and thermal effectiveness of latent heat storage systems. This paper, thus, aims to fill the research gap by developing a three-dimensional numerical model to study the effects of the fin number (i.e., 2, 4, 6, and 8) and fin length (i.e., 11.25, 17.00, and 22.00 mm) on the PCM melting/solidification characteristics and evaluate the performance and thermal effectiveness of the vertical double-tube latent heat storage system using the enthalpy-porosity method. Paraffin wax (RT-55) is used as the PCM, and water is the heat transfer fluid. The results show that increasing the fin number to eight reduces the melting, solidification, and total cycle time by 58.0%, 61.4%, and 59.8%, respectively, while the number of daily complete melting and solidification cycles increases by 150.0%, compared with the base case (i.e., without fins). The eight-fin storage system increases the daily charging capacity, thermal energy storage, thermal energy storage rate, and average effectiveness during melting by 146.0%, 133.8%, 123.5%, and 109.0%, respectively; while during solidification, the discharging rate, the daily discharging capacity, and average effectiveness increase by 159.9%, 149.3%, and 176.2%, respectively, compared with the base case. Increasing the fin length to 22.00 mm reduces the melting, solidification, and total cycle time by 80.3%, 81.3%, and 80.8%, respectively, while the number of daily complete melting and solidification cycles increases by 418.0%, compared with the base case. The highest fin length increases the daily charging capacity, thermal energy storage, thermal energy storage rate, and average effectiveness during melting by 370.0%, 350.0%, 330.0%, and 210.0%, respectively; during solidification, the discharging rate, the daily discharging capacity, and the average effectiveness increase by 420.0%, 410.0%, and 470.0%, respectively, compared with the base case.